[X] My Assistant

[Guest_Richard Trigaux_*]

James Oberg Chimes in:
Let Us Drink from the Fountains of Enceladus

The idea is as follows: send a stardust-like mission to capture particules from the Enceladus plume.

I think it is the cheapest way to have infos on what is going on into Enceladus, if it has a biochemistry and how far it evolved.

I allow myself to better the idea: the probe could have three targets.


1) when passing near Saturn, catches ring particules
2) Using Titan as a gravitationnal aid, captures smog particules (fortunately they reach very high)
3) passes into Enceladus plume, captures eventual evidences of biochemistry in Enceladus.

And back to Earth! It even don't need to actually satellise around Saturn, if it is well aimed. The only difficulty is a precise navigation, to aim into a 10x10kms window into the plumes, and a bit of fuel.

The only serious problem is not to bring back some alien bacteria on Earth! Eventually the aerogel containing the Enceladus particules would be coated in something after use, so that there would be no possible contamination, in any way.

Even if the mission fails, a low pass over the Tiger Stripes would allow to obtain precise images of the vents (or more likely zones where the ice is sublimating, like in a comet), provided we have a special shutter compensating for motion blur. Such images would be anyway a necessary preliminary step before sending a lander on Enceladus, and even before designing it (depending on the geometry of the vents, which may be complicated or hazardoous).

[Guest_RGClark_*]

Perhaps we could collect some of the 1.6% methane in the plumes to use as propellant. But you would need oxidizer. There is no oxygen in the plumes.
There is *very* tenuous molecular oxygen around Saturn rings. My guess is this would not be enough.
Is there a low energy method to get an oxidizer out of the water, nitrogen, and carbon monoxide in the plumes? (Separating oxygen out of H2O is energy intensive.)


Bob Clark

[Guest_RGClark_*]

Perhaps we could collect some of the 1.6% methane in the plumes to use as propellant. But you would need oxidizer. There is no oxygen in the plumes.
There is *very* tenuous molecular oxygen around Saturn rings. My guess is this would not be enough.
...

This article in Science gives the estimated amount of molecular oxygen above the Saturn A ring:

Oxygen Ions Observed Near Saturn's A Ring.
J. H. Waite, Jr., T. E. Cravens, W.-H. Ip, W. T. Kasprzak, J. G. Luhmann, R. L. McNutt, H. B. Niemann, R. V. Yelle, I. Mueller-Wodarg, S. A. Ledvina, S. Scherer
Science, 25 February 2005: Vol. 307. no. 5713, pp. 1260 - 1262
http://www.sciencemag.org/cgi/content/abstract/307/5713/1260

It estimates the number of neutral O2 molecules as 10^4 to 10^5 cm^-3. However, they note it could be much higher than this range because of the limitations of the measurements. I'll take the upper number, 10^5 cm^-3. There are 10^15 cubic centimenters in a cubic kilometer so this amounts to 10^20 molecules per km^3.
The article gives the orbital velocity around Saturn at the radial distance of the A ring as in the range of 15 km/s. Actually the article explains there are magnetic effects that accelerate the various ionized molecules even faster which when exchanging momentum with the neutral molecules accelerate these faster as well. I'll use the 15 km/s number for simplicity. Then if we orbit the spacecraft in the opposite direction we would have a relative velocity with respect to these molecules in the range of 30 km/s. So if we had a scoop with a 1km x 1km opening we could collect 30 x 10^20 molecules of O2 per second. To calculate the mass of this oxygen, Avogadro's number of O2 molecules, 6.0 x 10^23, amounts to 32 grams. So 30 x 10^20 molecules is 5 x 10^-3 moles or (5 x 10^-3) x 32 grams = 160 x 10^-3 grams. This is the mass collected every second. There are 31,536,000 seconds in a year, so after a year we would have 31,536,000 x 160 x 10^-3 grams = 5,045,760 grams, or 5045.76 kilos of O2.

This article gives the density of water molecules around Enceladus:

Enceladus Eruptions Enceladus Eruptions
Larry W. Esposito Larry W. Esposito
Principal Investigator Principal Investigator
UV imaging Spectrograph UV imaging Spectrograph
http://saturn.jpl.nasa.gov/multimedia/prod...RM_Esposito.pdf

On page 21 is given the density of water molecules versus altitude. The greatest density shown is about 10^7 molecules per cubic centimeter at 200 km altitude. The methane is 1.6 percent of this amount, so 1.6 x 10^5 molecules/cc. However, you couldn't move at high velocity around Enceladus such as 15 km/sec because the orbital velocity around it is so low.
One possibility would be to put it at one of the two Lagrange points that is close to the satellite. The distance from the surface would be higher than 200km so the density of the methane would be lower. You would need pumps then to draw in the methane. You would also have to separate out the more prevalent water from the methane.

Getting back to the molecular oxygen found around the rings, the researchers also found monoatomic hydrogen and monoatomic oxygen. They give the amounts of the ionized versions, but not the larger neutral amounts.
Both of these are known to be very efficient for propulsion: pure monoatomic hydrogen has about 3 to 4 times the ISP (specific impulse) as H2 with O2 as an oxidizer. A problem though that still has not been solved is how to store the monoatomic propellants stably within a rocket.
If all you wanted to do was to accelerate the rocket from Saturn then you could just use the monoatomic hydrogen as it normally combusts by bringing the single atoms together. That is, you would not need to store it. You couldn't use it though as an onboard propellant to engage on trajectory changes or landings and launches form satellites.


Bob Clark